1. Field of the Invention
The invention relates to a circuit for controlling the deployment of a safety cushioning device for occupants of a vehicle, often known as a passive restraint. More particularly, the circuit is for deploying an airbag within a steering wheel of an automobile.
2. Description of the Relevant Art
Control circuits for airbags are known from the German patent application DE-OS 195 39 070. The airbag system is located in the rotating part of the vehicle's steering wheel. A control device for activating an ignition impedance to release the airbag is located in a stationary part of the vehicle, e.g. in the steering column. The control information and the energy required for releasing the airbag is transmitted inductively by means of a transducer, such as a transformer. For providing an unintentional release, only a small energy amount is transmitted. A means for storing energy is provided in the steering wheel of the vehicle for storing sufficient energy for the ignition impedance. The means for storing energy can be for example a condenser.
A small power AC-signal is continuously transmitted to the device in the steering wheel, wherein the AC-signal is rectified and provides a constant energy supply. A trigger signal is sent for releasing the release unit, which trigger signal can be distinguished. from the AC-signal by a different frequency. The trigger signal is recognized by a control device switching a transistor for releasing the ignition impedance.
Further, the control circuit is controlling the charge state of the condenser and the function of the release unit. The resistance of the ignition impedance is measured for controlling the release unit. An error signal is send to the control device, if a defined value of the resistance is exceeded. Unfortunately, the error signal is not sent when a circuit failure in the steering wheel part of the circuit occurs, so that neither the circuit failure nor a failure of the ignition impedance is recognized.
In the German patent application DE-OS 195 30 586 Al a circuit for controlling the resistance of an ignition pill of an airbag is disclosed which is connected on the secondary side of a transformer. A failure of the airbag. function can be detected by a change in the resistance of the ignition pill. Therefore, the resistance is measured continuously and repeatedly for control. There is no unambiguous connection between the measured resistance and the resistance of the ignition pill, when the measurement is taking place at the primary coil of the transformer. It is proposed to connect an additional resistor in parallel to the ignition pill.
A further development of said circuit is described in the German patent application DE-OS 195 30 587 Al wherein a measurement resistor can be alternatively connected in addition to the ignition pill by use of a switch. The current at the primary side of the transformer is changing when said additional measurement resistor is connected to the ignition pill. The change in the current is required for functioning of an airbag and can be analyzed by a threshold gate.
In the German patent application DE-03 195 30 58 Al a further implementation of said circuit is described, wherein an impulse control charge is supplied to the primary side of the transformer for controlling the ignition impedance. The measurement is carried out by analyzing the rising and/or the falling time constant of the voltage being measured at the measurement resistor.
The problem in the above circuits is that the voltage which is measured is in conflict with the signals for controlling and releasing the airbag, when said signals are transmitted from the primary side to the secondary side of the transformer.